CHAPTER XXI. 

 FERTILIZATION AND DEVELOPMENT OF THE SEED.* 



314. Fertilization consists in the union of two cells, or of the 

 nuclei of two cells of a different nature, a sperm or male nucleus 

 with an egg or female nucleus. The result is a stimulus or im- 

 pulse given to the fertilized egg to develop and form an embryo 

 plant which later can develop into a plant like the parent. The 

 male nucleus, or sperm nucleus, is derived from the pollen grain 

 which is formed in the anther of the stamen. The egg is devel- 

 oped in the embryo sac within the ovule, which is in turn formed 

 in the ovary, a part of the pistil. 



315. The formation of the pollen, the sperm cells, and 

 pollen tube. The pollen is formed, as we have seen, in the 

 anther. When the pollen grain is very young it 



consists of a free cell in the pollen sac. The cell 

 wall encloses the protoplasm (paragraphs 17, 56) 

 the living substance, and within this is a very 

 important organ of the cell, the nucleus, which is 

 a more or less rounded body, finely granular and Fig. 150. 



Nearly mature 



usually appearing denser than the protoplasm, pollen grain of trii- 



. , . , Hum. The smaller 



As the pollen grain ripens a change takes place ceil is the genera- 

 in its contents as follows: The nucleus divides 

 into two nuclei, and very often a curved thin cell wall is formed 

 separating a small mass of the protoplasm with one nucleus from 



* This chapter is fcr reference or special assignment. The study of the 

 processes of fertilization and the development of the seed requires special 

 preparation of material and the use of technical methods which could not be 

 employed by students in the first-year course. It is desirable, however, 

 that students should have a general knowledge of these processes. This 

 chapter is presented for this purpose. It can be supplemented, if desirable, 

 by demonstrations of microscopic material prepared by the teacher or pur- 

 chased for the purpose. 



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